Self-focusing/compressing ultrashort pulse lasers are employed to generate acoustic and light sources that can acoustically, optically, and spectrally characterize underwater objects and environments and also be used to transmit data. The disclosed technology comprises a technique to characterize undersea objects and environments in ways that have never before been possible. The technique combines very short acoustic and optical pulses which provide broad-band “illumination” over the full white light optical spectrum, as well as over a very broad acoustic range, up to several Megahertz. In addition to the spectral analyses/imaging of objects/environment made possible by white-light illumination, a target material can be ablated, generating an ionized plume to spectrally identify the target's constituent atoms. This approach combines a number of cutting edge technologies, each of which has been demonstrated to some extent in different environments or with different laser pulses. As a result, although the technologies are complex and involve extension into new regimes, each element is grounded in past experiments, and it is their combination here and application in new environments that constitutes the primary advance.
Characterization of the environment and objects is often performed using acoustic-imaging techniques which involve “illuminating” the targeted scenes with large amounts of acoustic energy, centered around relatively low frequencies, with relatively narrow bandwidths. Optical characterization can also be performed, but again typically requires large amounts of illumination energy, especially considering the stronger attenuation in the ocean of optical energy than acoustic energy. There are several problems with illuminating the undersea environment with large amounts of energy. From a militarily tactical perspective, this practice generates a strong signature, advertising the illuminator's presence and allowing adversaries to much more easily detect, and then evade and/or find them. From an environmental perspective, depositing large amounts of energy into the ocean can damage the sea life/environment, resulting in unwanted effects and repercussions. A further technological advantage is that the broad-band, short acoustic and optical pulses will allow much greater resolution than the relatively long and narrow-band illumination pulses currently employed. Rastering high repetition-rate pulses to form a spatial array will allow for yet greater acoustic resolution, while time-gating the measured return signals (acoustic and/or optical) will provide much greater spatial resolution and penetration through turbid waters.
Combining ultrashort pulse lasers and short-gate imaging, Zevallos et al. have demonstrated the ability to resolve images through murky scattering environments, which were formerly completely impenetrable using any other optical means (Manuel E. Zevallos L., S. K. Gayen, M. Alrubaiee, and R. R. Alfano, “Time-gated backscattered ballistic light imaging of objects in turbid water”, Appl. Phys. Lett. 86, 011115 (2005)). Employment of femtosecond continuums adds a spectral element, not only allowing additional diagnostics (by seeing the spectrally-resolved return signals), but also ensures the presence of the least attenuated wavelength(s) for any given environment, exceeding those of state-of-the-art underwater LIDAR systems, which employ longer, monochromatic laser pulses. The shorter acoustic and optical pulse widths can enable increases in resolution of up to three orders of magnitude, and the increased penetration capability and time-gated imaging is anticipated to increase range by at least one to two orders of magnitude. The materials discrimination capabilities, made possible by laser-induced breakdown spectroscopy (A. Michel, M. Lawerence-Snyder, S. M. Angel, A. D. Chave “Oceanic Applications of Laser Induced Breakdown Spectroscopy: Laboratory Validation”, 2005 IEEE/MTS Annual Meeting); comparison of differently-filtered images, and bio-mimetic signal processing of broadband acoustic return signals, is a yet further benefit, which will allow an entirely new capability in target identification/discrimination.
Mullen et al. (L. J. Mullen, P. R. Herczfeld, and V. M. Contarino, IEEE Trans. Microwave Theory Tech. 44, 2703 (1996)) and Strand et al. (M. P. Strand, in Detection Technologies for Mines and Minelike Targets, edited by A. C. Dubey, I. Cindrich, J. M. Ralston, and K. Rigano [Proc. SPIE 2496, 487 (1995)]) have clearly articulated the need for new technologies to increase range and resolution in performing shallow-water surveying and underwater mine detection in turbid waters. The effectiveness of the employed techniques determines which waters can and cannot be mapped/characterized in advance, and once in a given environment, the ability to detect and characterize dangers ahead of a craft places constraints on speed and the ability to maneuver. Positive identification of obstacles is furthermore required to eliminate the need to treat debris the same way one treats a mine. Beyond operation in the field, the need for these capabilities is further required to help counter the increasing asymmetric threat coming from terrorist activities both abroad and at home. If the proposed approach increases range, resolution, and certainty by one to three orders of magnitude, it will allow a vehicle to proceed more quickly by the same amount when probing for dangers. For example, a 10-fold increase will allow an increase from 3 knots to 30 knots, which is operationally very significant. These capabilities are of great interest to both the United States Coast Guard and the United States Navy, as well as to merchant, commercial, and private vessel operators.
The disclosed illumination approach addresses a number of current operational problems faced by the military, including characterization of the littoral environment and identification of mines, unexploded ordnance, and environmental impact/effects. This optical and acoustic characterization is of particular importance in submarine situational awareness, since it will provide high quality imaging and enhancement of collision avoidance capability. Targeted high-resolution acoustic imagery is often difficult to obtain in a complex environment, and optical characterization can be difficult or impossible to obtain with monochromatic sources and especially in turbid waters. A direct benefit of the disclosed technique will be enhanced remote sensing/detection of ordnance in near-shore locations. A further benefit of the spectral aspect of the technology is its potential use as a tool by both the US Navy and Coast Guard for pollution/HAZMAT prevention and response decision. Not only will the spectral analysis ability help in material identification, but it can also be coupled to software and databases for automation of this task.
The claimed technology is a multifaceted tool to address several problems, which are currently approached in a number of ways. Identification of chemicals is often done through water sampling and chemical analysis, which is a time-consuming and cumbersome process. Identification of a target material is often done through visual imagery and assessment of the material's acoustic impedance-mismatch with water. This can be an uncertain process and is susceptible to deception techniques. In many cases, positive identification requires close proximity to the target, with the best diagnosis involving the deployment of a diver. However, close proximity of divers and assets is undesirable when assessing the nature of a given target object. Maintaining a large stand-off distance and illuminating the object with white light and acoustic energy currently requires a large flash and loud ping. However, the nature of maritime scatterers tends to diffuse this input energy and “blur” the final results.
Another approach to characterizing targets and the maritime environment is to obtain spectral information. Performing these measurements on the water column allows the assessment of its chemical content, however conventional means to acquire spectral information require either direct sampling or a remote white light source that points through the targeted medium to a spectrometer. These approaches can be time-consuming and risky because of the proximity of either a vehicle and/or tether. Direct spectral analysis of a solid target using conventional techniques also requires physical contact with the target. Again, this is inherently dangerous, and the identified concerns in these prior capabilities are obviated through the disclosed method.
The coupling of electromagnetic/optical probing techniques to acoustic signals presents the potential to decouple the observer from the water, which has sparked significant interest and extensive research. One current worldwide effort is to increase the Maritime Domain Awareness. An important goal for this program is to develop passive acoustic sensors which can be liberally deployed, measure a wide range of signals, and require little to no power or maintenance. Employing optical techniques to probe the acoustic environment represents an approach with many benefits, in that signals can be emitted and measured from outside the water, without depending on assets in the water. Two of the key organizations involved in this effort are the U.S. Coast Guard (USCG) and the National Oceanic and Atmospheric Administration (NOAA). Their interest in this application is further reason to pursue the investigation of coupled optical and acoustic measurements.
The ability to remotely create an acoustic signal in the water has been investigated using both continuous wave and pulsed lasers by a number of Department of Defense (DoD) agencies. The Naval Undersea Warfare Center—Newport Division has worked on developing laser acoustic source and detection schemes at the ocean surface in order to communicate with undersea vehicles. Their approach allows for minimal air propagation, requires lens focusing, and takes advantage of neither underwater optical propagation nor the remote optical compression enabled by ultrashort pulse lasers. Their modulated continuous wave laser acoustic source arrays at the ocean surface also do not benefit from underwater laser propagation. The methods furthermore yield undesirably low efficiencies and weak acoustic signals because the mechanism to generate acoustic signals when using low laser intensities involves heating instead of optical breakdown. To investigate the benefits of optical breakdown, the Naval Research Laboratory (NRL) has had several groups investigate this technique to generate acoustic signals using high-intensity pulsed lasers. Certain groups have investigated short-pulse lasers (nanoseconds) with pulse energies exceeding 100 Joules (J), while other groups have investigated ultrashort laser pulses (sub-picosecond pulses) with millijoules (mJ) of energy per pulse. Their acoustic results are sufficiently pertinent that we have included some of them in the descriptions of section B. They have recently received a U.S. Pat. No. 7,260,023 for the generation of acoustic signatures using non-linear self-focusing, with an ongoing application (20060096802).
S. V. Egerev describes development of noncontact laser acoustic sources in “In Search of a Noncontact Underwater Acoustic Source”, Acoustical Physics, vol. 49, issue 1, pages 51-61, 2003. A laser-based ultrasonic and hypersonic sound generator is discussed in U.S. Pat. No. 3,392,368 to Brewer et al. Laser induced electric breakdown in water is discussed by C. A. Sacchi in the Journal of the Optical Society of America B, Vol. 8, No. 2, February 1991, pages 337-345. P. K. Kennedy discusses laser induced breakdown thresholds in ocular and aqueous media in IEEE Journal of Quantum Mechanics, Vol. 31, No. 12, December 1995, pages 2241-2249 and 2250-2257. A. Vogel and S. Busch discuss shock wave emission and cavitation generation by picosecond and nanosecond optical breakdown in water in J. Acoustical Society of America, Vol. 100, Issue 1, July 1996, pages 148-165.
T. G. Jones, J. Grun, L. D. Bibee, C. Manka, A. Landsberg, and D. Tam discuss laser-generated shocks and bubbles as laboratory-scale models of underwater explosions in Shock and Vibration, IOP Press, Vol. 10, pages 147-157, 2003.
P. Sprangle, J. R. Penano, and B. Hafizi discuss propagation of intense short laser pulses in the atmosphere in Physical Review E, Vol. 66, 2002, pages 046418-1-046418-21. The optical Kerr effect, a non-linear change in the refractive effect at high intensity, is discussed by Siegman, Lasers, pages 375-386, 1986.